1. Field of the Invention
The present invention relates to a thin film magnetic head and magnetic disk device having an alternating-current drive function in a microwave band as an assist method for writing data signals on a magnetic recording medium having a large coercive force for stabilizing magnetization.
2. Description of the Related Art
With advancements in high density recording, bit cells for digital information recorded onto magnetic recording media have become finer. As a result, so-called thermal fluctuation occurs with which signals detected by the reproducing element of a magnetic head fluctuate, whereby the S/N (signal to noise ratio) may deteriorate or signals may even disappear in the worst case.
Therefore, recently, it is supposedly an effective solution to the above problem to increase the perpendicular magnetic anisotropic energy Ku of a recording film in currently used magnetic recording media using perpendicular recording systems. The thermal stability index S corresponding to thermal fluctuation is expressed by Equation 1 below. It is generally considered that the value of S must be 50 or higher.S=Ku·V/kB·T  Equation (1)
In Equation (1),                Ku: perpendicular magnetic an isotropic energy;        V: cubic volume of the crystal grains composing the recording film;        kB: Boltzmann constant; and        T: absolute temperature.        
According to a so-called Stoner-Wohlfarth model, the anisotropic magnetic field Hk and coercive force Hc of a recording film is expressed by Equation (2). The coercive force Hc is increased as Ku is increased (However, Hk>Hc in conventional recording films).H=Hc=2Ku/Ms  Equation (2)where Ms is the saturation magnetization of the recording film.
A steep recording magnetic field up to approximately Hk must be applied for magnetization reversal of a recording film corresponding to a desired data series. Magnetic disk devices recently made available for practical use (HDD) using perpendicular recording systems have a recording element utilizing a so-called magnetic monopole, in which a recording magnetic field is applied from the ABS (air bearing surface) to the recording film in its perpendicular direction.
The intensity of the perpendicular recording magnetic field is proportional to the saturation magnetic flux density Bs of the soft magnetic material forming the magnetic monopole. Therefore, materials having a saturation magnetic flux density Bs as high as possible have been developed and are in practical use. However, the saturation magnetic flux density Bs is considered to have a practical upper limit Bs=2.4 T (tesla) from the so-called Slater-Pauling curve and is currently believed to be approaching such a practical limit. The thickness and width of existing magnetic monopoles is approximately 100 to 200 nm. In order to increase the recording density, the thickness and width must be reduced while the generated perpendicular magnetic field tends to be diminished accordingly.
For the above reason, the recording performance of conventional data writing elements is approaching its limit and high density recording has recently become increasingly difficult.
Therefore, so-called thermal assisted magnetic recording (TAMR) is proposed in which the recording film is irradiated and heated with laser light or the like to reduce the coercive force of the recording film before recording signals.
However, the thermal assisted recording has the following problems:
(1) the structure is extremely complicated and expensive as it requires a magnetic head on which a magnetic element and an optical element are mounted; (2) a recording film having largely varied temperature characteristics of the coercive force should be developed; and (3) adjacent track erasing and an unstable recording state occur due to thermal demagnetization during the recording process.
Meanwhile, in accordance with giant magnetoresistive elements (GMR elements) and tunneling magnetoresistance elements (TMR elements) becoming more sensitive, active research has been conducted on spin transfer in electron conduction. Further, a research is conducted to apply the above research to magnetization reversal of the recording film of a magnetic recording medium so that the perpendicular magnetic field necessary for magnetization reversal is reduced. A technical proposal relating to such research is to directly form a microwave generating element as a lamination film between the main pole and auxiliary pole. Then, a high frequency alternating-current magnetic field is applied in the in-plane direction of the magnetic recording medium concurrently with the perpendicular magnetic field.
The frequency of the alternating-current magnetic field applied in the in-plane direction is an ultra high frequency (10 GHz to 30 GHz) in a microwave band that corresponds to the ferromagnetic resonant frequency of recording films. The alternating-current magnetic field applied in the in-plane direction is supposedly able to reduce the required reversal magnetic field in a perpendicular direction to approximately 60% of Hk. If this technology comes into practical use, there will be no need to use the aforementioned complicated HAMR while making it possible to raise the Hk of a recording film, thereby giving rise to the expectation of significant improvement in recording density.
However, in the above proposal, since it is required that a microwave generating element is directly formed as a lamination film between the main pole and auxiliary pole, the manufacturing method is extremely difficult. Therefore, the inventors of the present application have proposed a mechanism in which a microwave excitation current supply circuit is provided outside the magnetic head and a microwave excitation current supplied from the microwave excitation current supply circuit and a direct-current excitation current are guided to a microwave radiator provided in the thin film magnetic head to generate a high frequency magnetic field from the vicinity of the recording part of the thin film magnetic head. In other words, as a microwave radiation means in a microwave assisted magnetic recording system, a waveguide having a planar structure of an inverted microstrip waveguide (I-MLIN) is used to obtain a microwave radiating body having a rectangular cross-section with a preferable aspect ratio (JP Application No. 2009-030937 filed on Feb. 13, 2009).
The invention of the present application further improves the above proposed technology of the inventors of the present application, and aims to further improve the microwave assisted effect by tilting a microwave magnetic field generated from a microwave radiator in an orientation manner directly below the main pole of the writing magnetic pole.